Compensation of volatile-distributed current due to variance of the unknown contact impedance in an electrical impedance tomography sensor

Abstract

A weighted parameter vector χnorm based on extracellular fluid resistance in the Cole impedance model is hereby proposed in order to compensate for the volatile-distributed current. The volatile-distributed current due to variance in the unknown contact impedance in an electrical impedance tomography (EIT) sensor distorts the ideality of the frequency-dependent behavior of the object of interest, leading to inaccuracy in the reconstructed images. The sources of variance in the unknown contact impedance include the use of electrodes with inconsistent impedance values, electrochemical reaction or the improper attachment of electrodes to the object of interest. χnorm represents the current pathway lengths at zero frequency which reflects the volatile-distributed current, and it is used to determine the source of the measured impedance. The source of the unknown contact impedance can be the background object in the normal physiological condition, an inclusion object in an abnormal physiological condition or systematic error. The new reconstruction methods are derived from the frequency-difference EIT (fd-EIT) and the weighted-frequency-difference EIT (wfd-EIT) using χnorm. The new reconstruction methods are frequency-difference electrical impedance spectro-tomography (fd-EIST) and weighted-frequency-difference electrical impedance spectro-tomography (wfd-EIST). The performance of fd-EIST and wfd-EIST was evaluated by experimental and simulation studies using biomaterials with frequency measurements from f = 500 Hz to f = 100 kHz and the results were compared with those from fd-EIT and wfd-EIT. The results showed that the use of χnorm reduces the root mean square error, position error, ringing and shape deformation. It also increases the ideality of the frequency-dependent behavior, either for the inclusions or the background objects at different frequencies.